Origin of the Positive Temperature Coefficient of Resistivity Anomaly in the ZnO‐NiO System

An anomalous positive temperature coefficient of resistivity (PTCR) was investigated in the ZnO-NiO system. It was found that the ZnO ss (Zn 0.97 Ni 0.03 O) and NiO ss (Ni 0.6 Zn 0.4 O) constituent phases of that system exhibit negative temperature coefficient of resistivity (NTCR) character, while their combination shows a PTCR effect with a maximum at 400°C, which coincides with a large difference in the coefficient of linear thermal expansion between the ZnO ss and NiO ss phases at that temperature. On the basis of the brick wall model microstructure, the PTCR anomaly of this system can be explained. The magnitude of the PTCR effect is governed by the difference in resistivity of the two constituent phases at the temperature where the maximum of the PTCR anomaly occurs. The predicted temperature dependence of the resistance, R(T), of a model microstructure consisting of constituent phases with different grain sizes agrees well with the experimental R(T) of the prepared composite ceramics.